1. Field of the Invention
The present invention relates to an interface circuit. More specifically, the invention relates to an interface circuit that receives a binary signal through a capacitor.
2. Description of Related Art
Traditionally, when different semiconductor devices are connected, connection of the semiconductor devices through a capacitor has been often performed. The capacitor has a function of passing an AC component of an input signal and blocking a DC component of the input signal. For this reason, by connecting the capacitor in series with a signal line that connects the semiconductor devices having mutually different bias voltages of an AC signal to be handled, a difference between the bias voltages may be eliminated by the capacitor. Japanese Unexamined Patent Application Publication No. 2001-57546 discloses a method of connecting the semiconductor devices using the capacitor as described above.
The capacitor, however, has a characteristic in which a voltage at a terminal of the capacitor connected to a semiconductor device on a receiving side is reduced over timer according to a time constant determined by an input impedance of the semiconductor device on the receiving side and a capacitance value of the capacitor. For this reason, when the semiconductor devices are connected through the capacitor, it is necessary to set the capacitance value of the capacitor and the input impedance of the semiconductor device on the receiving side, according to a frequency of a signal to be handled.
When the frequency of the signal to be handled is extremely low (e.g. when a certain voltage level continues for several hours or longer), the capacitance value of the capacitor or the input impedance of the semiconductor device on the receiving side must be extremely increased. In such a case, the capacitance value of the capacitor and the input impedance of the semiconductor device on the receiving side may be set unfeasibly large.
It has been discovered that, using the technique described in Japanese Unexamined Patent Application Publication No. 2001-57546, it is impossible to normally perform communication between the semiconductor devices using the signal of the extremely low frequency while blocking a DC component of the signal generated between the semiconductor devices.